JET researchers are pleased as the 2011 – 2012 experimental period concludes

“We have ticked all the boxes for ITER” says Guy Matthews, project leader for the ITER-Like-Wall at JET. Sebastijan Brezinsek, who leads the JET Task Force responsible for the exploitation of the new wall, agrees: “We had to learn how to operate JET with the ITER-Like Wall” he says. “We have proved in the last experimental campaign that we can operate in the baseline H-mode scenario with high reproducibility, low disruption rate, and without tungsten events at all, even though operation is quite different to the carbon wall.” Brezinsek continues to explain: “The operational window for good confined H-mode is quite narrow, but we have learned how much fuelling and central heating is required to keep the divertor cool while still maintaining a minimum number of ELM events to flush the tungsten impurities.”

However, the cleaner plasma – without the carbon impurities that were ubiquitous with the previous wall tiles – has led to some unexpected behaviour. In the standard mode of operation, known as baseline, the confinement is not as good as in the best carbon references. Surprisingly, though, in the more advanced “hybrid” mode, the confinement with the ITER-Like Wall is comparable to that achieved with the carbon wall. ITER, however, will need to operate in both scenarios. “We will have to look into this,” says JET Close Support Unit operation group leader George Sips. “The confinement is probably less good because there is no beneficial radiation from the carbon in the plasma edge. Nobody would have expected that we need to make the plasma dirty to make it work better!” Initial experiments involving the seeding of the plasma with nitrogen to increase radiation appear to address the problem, but more experiments are needed to fully understand the situation.

Experiments finished with a two-week campaign in which 150 identical pulses were produced over and over again, in order to build up a total of 900 seconds of stable operation – the equivalent of one pulse at ITER. As well as proving that plasma could be held stable in a tokamak for that long – these experiments are designed to test the materials of the ITER-Like Wall, to see how they behave during long term operation. Various tungsten and beryllium tiles will be removed from the vessel during the current maintenance shutdown period and analysed closely. “We have already measured the gas balance, but this will give us a complementary information about the long-term fuel retention and help us to understand which mechanisms are responsible,” says Sebastijan Brezinsek.

These significant results for ITER come as the international organisation formalises its collaboration with JET by means of the IEA implementing agreement on Cooperation on Tokamak Programmes. Even before experiments had finished, the ITER Director General visited JET, to discuss future experiments, such as a deliberate melting of a number of tungsten tiles to see how this affects operation.

Phil Dooley, EFDA